The following text is from Joseph Conrad's 1907 novel The Secret Agent: A Simple Tale. Mr. Verloc is navigating the London streets on his way to a meeting.

Before reaching Knightsbridge, Mr. Verloc took a turn to the left out of the busy main thoroughflare, uproarious with the traffic of swaying omnibuses and trotting vans, in the almost silent, swift flow of hansoms [horse-drawn carriages]. Under his hat, worn with a slight backward tilt, his hair had been carefully brushed into respectful sleekness; for his business was with an Embassy. And Mr. Verloc, steady like a rock--a soft kind of rock--marched now along a street which could with every propriety be described as private.

Which choice best describes the function of the underlined phrase in the text as a whole?